Techniques for dual dielectric thickness for a nanowire CMOS technology using oxygen growth

ABSTRACT

In one aspect, a method of forming a CMOS device includes forming nanowires suspended over a BOX, wherein a first/second one or more of the nanowires are suspended at a first/second suspension height over the BOX, and wherein the first suspension height is greater than the second suspension height; depositing a conformal gate dielectric on the BOX and around the nanowires wherein the conformal gate dielectric deposited on the BOX is i) in a non-contact position with the conformal gate dielectric deposited around the first one or more of the nanowires, and ii) is in direct physical contact with the conformal gate dielectric deposited around the second one or more of the nanowires such that the BOX serves as an oxygen source during growth of a conformal oxide layer at the interface between the conformal gate dielectric and the second one or more of the nanowires.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 14/671,041filed on Mar. 27, 2015, now U.S. Pat. No. 9,536,794, the contents ofwhich are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to nanowire complementary metal oxidesemiconductor (CMOS) devices and more particularly, to techniques forcreating dual dielectric thickness for nanowire CMOS technology usingoxygen growth based on varying a suspension height of the nanowires overa buried oxide (BOX).

BACKGROUND OF THE INVENTION

Threshold voltage (Vt) is an important parameter to be able to controlin metal oxide semiconductor field effect transistor (MOSFET) devices.In bulk MOSFET designs wherein the channel is formed in a bulksemiconductor, the Vt is commonly adjusted through doping. It is oftendesirable to have multiple Vt's in a MOSFET design wherein the Vt variesfrom one device to another.

Setting multiple Vt's in a fully depleted technology presents manychallenges, as doping to adjust Vt is no longer an option. It has beenfound that the Vt of planar p-channel FETs can be adjusted throughoxidation of the high-κ gate dielectric. See for example, Cartier etal., “pFET Vt control with HfO2/TiN/poly-Si gate stack using a lateraloxygenation process,” 2009 Symposium on VLSI Technology, pgs. 42-43(June 2009) (hereinafter “Cartier”) and U.S. Patent ApplicationPublication Number 2009/0289306 A1 by Watanabe et al., entitled “LateralOxidation with High-k Dielectric Liner” (hereinafter “U.S. PatentApplication Publication Number 2009/0289306 A1”).

There however exists a need for controlling Vt in non-planar deviceconfigurations.

SUMMARY OF THE INVENTION

The present invention provides techniques for creating dual dielectricthickness for nanowire complementary metal oxide semiconductor (CMOS)technology using oxygen growth based on varying a suspension height ofthe nanowires over a buried oxide (BOX). In one aspect of the invention,a method of forming a CMOS device is provided. The method includes thesteps of: providing a semiconductor-on-insulator (SOI) wafer having aSOI layer separated from a substrate by a BOX; forming nanowiressuspended over the BOX, wherein a first one or more of the nanowires aresuspended at a first suspension height over the BOX and a second one ormore of the nanowires are suspended at a second suspension height overthe BOX, and wherein the first suspension height is greater than thesecond suspension height; depositing a conformal gate dielectric on theBOX and around the nanowires, wherein the conformal gate dielectricdeposited on the BOX is i) in a non-contact position with the conformalgate dielectric deposited around the first one or more of the nanowires,and ii) is in direct physical contact with the conformal gate dielectricdeposited around the second one or more of the nanowires; depositing aconformal gate metal layer on the conformal gate dielectric both on thewafer and on the nanowires, wherein the conformal gate metal layer fullysurrounds the first one or more of the nanowires but only partiallysurrounds the second one or more of the nanowires due to the conformalgate dielectric on the BOX being in direct physical contact with theconformal gate dielectric around the second one or more of thenanowires; depositing a conformal polysilicon layer on the conformalgate metal layer both on the wafer and on the nanowires; and annealingthe CMOS device in an oxygen ambient to grow a conformal oxide layer atan interface between the conformal gate dielectric and the nanowires,wherein the conformal oxide layer grown at the interface between theconformal gate dielectric and the first one or more of the nanowires hasa first thickness and the conformal oxide layer grown at the interfacebetween the conformal gate dielectric and the second one or more of thenanowires has a second thickness, and wherein the first thickness isless than the second thickness due to the BOX serving as an oxygensource during growth of the conformal oxide layer at the interfacebetween the conformal gate dielectric and the second one or more of thenanowires.

In another aspect of the invention, a CMOS device is provided. The CMOSdevice includes nanowires suspended over a BOX, wherein a first one ormore of the nanowires are suspended at a first suspension height overthe BOX and a second one or more of the nanowires are suspended at asecond suspension height over the BOX, and wherein the first suspensionheight is greater than the second suspension height; a conformal gatedielectric on the BOX and around the nanowires, wherein the conformalgate dielectric on the BOX is i) in a non-contact position with theconformal gate dielectric around the first one or more of the nanowires,and ii) is in direct physical contact with the conformal gate dielectricaround the second one or more of the nanowires; a conformal gate metallayer on the conformal gate dielectric both on the wafer and on thenanowires, wherein the conformal gate metal layer fully surrounds thefirst one or more of the nanowires but only partially surrounds thesecond one or more of the nanowires due to the conformal gate dielectricon the BOX being in direct physical contact with the conformal gatedielectric around the second one or more of the nanowires; a conformalpolysilicon layer on the conformal gate metal layer both on the waferand on the nanowires; and a conformal oxide layer at an interfacebetween the conformal gate dielectric and the nanowires, wherein theconformal oxide layer at the interface between the conformal gatedielectric and the first one or more of the nanowires has a firstthickness and the conformal oxide layer at the interface between theconformal gate dielectric and the second one or more of the nanowireshas a second thickness, and wherein the first thickness is less than thesecond thickness.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating a starting structurefor forming a nanowire complementary metal oxide semiconductor (CMOS)device including a semiconductor-on-insulator (SOI) wafer having a SOIlayer separated from a substrate by a buried oxide (BOX) according to anembodiment of the present invention;

FIG. 2 is a cross-sectional diagram illustrating a stepped surface ofthe SOI layer having been formed in a first and a second regions of thewafer according to an embodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating a stepped surface ofthe SOI layer having been formed in a third region of the waferaccording to an embodiment of the present invention;

FIG. 4 is a cross-sectional diagram illustrating a layer of asemiconductor material having been epitaxially grown on the SOI layeraccording to an embodiment of the present invention;

FIG. 5 is a cross-sectional diagram illustrating fins having beenpatterned in the epitaxial layer/SOI layer according to an embodiment ofthe present invention;

FIG. 6 is a cross-sectional diagram illustrating the SOI layer havingbeen removed from the fins selective to the epitaxial layer whichreleases the epitaxial layer from the fins, wherein the releasedepitaxial layer forms suspended nanowires of the device according to anembodiment of the present invention;

FIG. 7 is a cross-sectional diagram illustrating the nanowires havingbeen re-shaped, e.g., smoothed, to give them a circular cross-sectionalshape according to an embodiment of the present invention;

FIG. 8 is a three-dimensional diagram illustrating an exemplaryembodiment wherein both nanowires and pads have been patterned in theepitaxial layer according to an embodiment of the present invention;

FIG. 9 is a cross-sectional diagram illustrating a conformal gatedielectric having been blanket deposited on the wafer and surroundingthe nanowires, wherein a suspension height of the nanowires determineswhether the conformal gate dielectric (of a given thickness) does ordoes not physically contact the underlying BOX according to anembodiment of the present invention;

FIG. 10 is a cross-sectional diagram illustrating a conformal gate metallayer having been blanket deposited on the conformal gate dielectricboth on the wafer and around the nanowires according to an embodiment ofthe present invention;

FIG. 11 is a cross-sectional diagram illustrating a conformalpolysilicon layer having been blanket deposited on the conformal gatemetal layer both on the wafer and around the nanowires according to anembodiment of the present invention;

FIG. 12 is a cross-sectional diagram illustrating an anneal in an oxygenambient having been used to form a conformal oxide layer at theinterface between the conformal gate dielectric and the nanowireswherein the conformal gate dielectric, when in direct physical contactwith the BOX, permits the BOX to act as an additional oxygen sourceaccording to an embodiment of the present invention;

FIG. 13 is a cross-sectional diagram illustrating an alternativestarting structure for forming a nanowire CMOS device including a SOIwafer having a SOI layer (in which nanowires and pads will be formed)separated from a substrate by a BOX according to an embodiment of thepresent invention;

FIG. 14 is a cross-sectional diagram illustrating nanowires having beenpatterned in the SOI layer according to an embodiment of the presentinvention;

FIG. 15 is a cross-sectional diagram illustrating the nanowires havingbeen suspended over the BOX using an etch to undercut the BOX to a firstdepth beneath the nanowires in a first region of the wafer according toan embodiment of the present invention;

FIG. 16 is a cross-sectional diagram illustrating the BOX having beenundercut to a second depth beneath the nanowires in a second region ofthe wafer according to an embodiment of the present invention;

FIG. 17 is a cross-sectional diagram illustrating the BOX having beenundercut to a third depth beneath the nanowires in a third region of thewafer according to an embodiment of the present invention;

FIG. 18 is a cross-sectional diagram illustrating the nanowires havingbeen re-shaped, e.g., smoothed, to give them a circular cross-sectionalshape according to an embodiment of the present invention;

FIG. 19 is a three-dimensional diagram illustrating an exemplaryembodiment wherein both nanowires and pads have been patterned in theSOI layer according to an embodiment of the present invention;

FIG. 20 is a cross-sectional diagram illustrating a conformal gatedielectric having been blanket deposited on the wafer and surroundingthe nanowires, wherein a suspension height of the nanowires based on thedepth of the undercut BOX determines whether the conformal gatedielectric (of a given thickness) does or does not physically contactthe underlying BOX according to an embodiment of the present invention;

FIG. 21 is a cross-sectional diagram illustrating a conformal gate metallayer having been blanket deposited on the conformal gate dielectricboth on the wafer and around the nanowires according to an embodiment ofthe present invention;

FIG. 22 is a cross-sectional diagram illustrating a conformalpolysilicon layer having been blanket deposited on the conformal gatemetal layer both on the wafer and around the nanowires according to anembodiment of the present invention;

FIG. 23 is a cross-sectional diagram illustrating an anneal in an oxygenambient having been used to form a conformal oxide layer at theinterface between the conformal gate dielectric and the nanowireswherein the conformal gate dielectric, when in direct physical contactwith the BOX, permits the BOX to act as an additional oxygen sourceaccording to an embodiment of the present invention;

FIG. 24 is a cross-sectional diagram illustrating the present devicehaving multiple nanowires at a given suspension height over the BOX suchthat the gate dielectric is not in direct physical contact with the BOXand therefore minimal oxide growth occurs at the interface between theconformal gate dielectric and the nanowires according to an embodimentof the present invention; and

FIG. 25 is a cross-sectional diagram illustrating the present devicehaving multiple nanowires at a given suspension height over the BOX suchthat the gate dielectric is in direct physical contact with the BOX andtherefore an increased amount of oxide growth occurs at the interfacebetween the conformal gate dielectric and the nanowires according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are techniques for controlling the threshold voltage(Vt) of nanowire complementary metal oxide semiconductor (CMOS) devicesusing a gate first process wherein a suspension height of the nanowiresover the buried oxide (BOX) is modulated to control whether or not thehigh-κ gate dielectric in the present polysilicon/metal/high-κ gatestacks contacts the BOX. The term “high-κ” refers to a material having adielectric constant κ which is higher than that of silicon (i.e., 11.7).When there is contact between the high-κ gate dielectric and the BOX, agreater amount of oxide growth occurs at the interface between the gatedielectric and the nanowires. Namely, the BOX in that case acts as anadditional source of oxygen which diffuses through the high-κ gatedielectric. Oxidation reduces the Vt. See for example, Cartier and U.S.Patent Application Publication Number 2009/0289306 A1, the contents ofeach of which are incorporated by reference as if fully set forthherein. Being able to regulate the amount of oxidation permits one tothereby regulate the amount by which the Vt is lowered. For instance,when the high-κ gate dielectric and the BOX are not in contact (such aswhen the metal gate stack layers are present therebetween—see below)then the amount of growth at the gate dielectric-nanowire interface isminimal.

A first exemplary embodiment of the present techniques is now describedby way of reference to FIGS. 1-12 which illustrate an exemplary processfor forming a nanowire CMOS device. In this example, a variation in thesuspension height of the nanowires over the underlying BOX is achievedusing a stepped surface of a sacrificial material on an insulator waferto produce nanowires at varying heights above the wafer. Gate stackswill then be formed as a series of conformal layers around thenanowires. The suspension height of the nanowires controls the spaceunder the nanowires for deposition of the gate stack materials andultimately whether or not the high-κ gate dielectric in the gate stackis in contact with the insulator.

As shown in FIG. 1, the process begins with a semiconductor-on-insulator(SOI) wafer. Specifically, the SOI wafer includes a SOI layer 106separated from a substrate 102 by a BOX 104 (e.g., silicon dioxide(SiO₂)). In the present example, the SOI layer 106 will be used toprovide a stepped surface onto which the nanowires can be formed. TheSOI layer 106 will subsequently be removed to suspend the nanowires.Thus, the SOI layer 106 in this case is a sacrificial layer.Accordingly, the SOI layer is preferably formed from a material that canbe easily removed selective to the nanowires. By way of example only, ifthe nanowires are silicon (Si) (see below), then a suitable sacrificialmaterial for forming SOI layer 106 includes, but is not limited to,silicon germanium (SiGe). In that case, the starting wafer is aSiGe-on-insulator wafer.

Next, a series of masking and etching steps are used to create a steppedsurface on a side of the SOI layer 106 opposite the BOX 104. See FIGS. 2and 3. It is notable that the thickness of the SOI layer 106 willcontrol the suspension height of the nanowires (i.e., the height atwhich the nanowires are suspended over the BOX). The stepped surfacevaries the thickness of the SOI layer in different regions of the wafer.Thus, the starting thickness of the SOI layer 106 should be at least asthick as the greatest suspension height desired. For illustrativepurposes only, the various regions of the wafer in which differentnanowire suspension heights will be created are labeled as region I,region II, etc.

According to an exemplary embodiment, the stepped surface of the SOIlayer 106 is created by masking the SOI layer in a first region(s) ofthe wafer (labeled “region I”) and using an etching process to reducethe thickness of the SOI layer 106 in one or more other regions of thewafer. See FIG. 2. As shown in FIG. 2, a mask 202 is formed on the SOIlayer 106 in the first region(s) (region I) of the wafer. Mask 202 maybe formed using a standard lithography and etching process. A timed etch(using for example reactive ion etching (RIE)) is then used to removeunmasked portions of the SOI layer 106. The etch is endpointed when the(unmasked) portions of the SOI layer 106 reach a desired (reduced)thickness. As highlighted above, the goal of this step etch is tocontrol the suspension height of the nanowires, and ultimately toregulate whether or not the gate stack materials fully surround thenanowires. Thus, the desired thickness of the SOI layer 106 is directlyrelated to the desired nanowire suspension height.

At this point in the process, the SOI layer 106 (via the steppedsurface) now has regions with two different thicknesses. One could nowmove on to form the nanowires over this stepped surface. However, inorder to further illustrate the present process for forming a steppedsurface, a region of the SOI layer having a third thickness is created.See FIG. 3. Specifically, as shown in FIG. 3 a mask 302 is formed on theSOI layer 106 in a second region(s) (region II) of the wafer. Again,mask 302 may be formed using a standard lithography and etching process.Mask 202 (covering/masking the first region(s) (region I) of the wafercan remain in place. A timed etch (using for example RIE) is then usedto remove unmasked portions of the SOI layer 106. The etch is endpointedwhen the (unmasked) portions of the SOI layer 106 (that are in this casein a third region(s) (region III) of the wafer) reach a desired(reduced) thickness. Following completion of the stepped surface etch,any remaining hardmask may then be removed.

As shown in FIG. 3, the result of the step surface etch is the SOI layerhaving different thicknesses in different regions of the wafer. In thisparticular example, the SOI layer 106 has three different thicknessesT1, T2, and T3 in regions I, II, and III of the wafer, respectively. Asprovided above, the thickness of the SOI layer 106 controls thesuspension height of the nanowires, i.e., the height at which thenanowires will be suspended over the BOX after the SOI layer 106 isremoved. Thus, in this particular example, nanowires will be producedthat are suspended over the BOX at three different heights (H1, H2, andH3—see below).

The terms “first,” “second,” “third,” etc. may be used throughout thepresent description to distinguish the nanowires, the various differentsuspension heights, BOX undercut depths (see below), layer thicknesses,etc. For instance, according to the present techniques, one or morefirst nanowires will be formed at a first suspension height H1 over theBOX, one or more second nanowires will be formed at a second suspensionheight H2 over the BOX, etc., wherein H1 is greater than H2 (i.e.,H1>H2), etc. The various first, second, etc. suspension heights may beachieved, for example, using the stepped SOI having a first thickness, asecond thickness, etc., or by undercutting the BOX (see below) to afirst depth, a second depth, etc. As noted above, it is not requiredthat the same number of suspension heights, thicknesses, depths, etc. becreated as shown (for example, embodiments are anticipated herein whereonly two different suspension heights are employed rather than, e.g.,the three shown.

To begin the nanowire formation phase of the process, a layer 402 of asemiconductor material is epitaxially grown on the SOI layer 106. SeeFIG. 4. As provided above, since the SOI layer 106 will serve as asacrificial release layer that is removed in order to suspend thenanowires, the materials for forming the SOI layer 106 and the nanowiresshould be chosen to enable removal of the SOI layer selective to thenanowires. By way of example only, a SiGe SOI layer 106 and Si nanowiresis a suitable combination. Thus, according to an exemplary embodiment,epitaxial Si is grown as layer 402 on the stepped surface of the SOIlayer 106.

As shown in FIG. 4, the epitaxial layer 402 of semiconductor material ispreferably grown to a uniform thickness T_(EPITAXY) on SOI layer 106.This will result in nanowires being produced having the samecross-sectional shape and dimensions. However, due to the SOI layer 106having a stepped surface, the nanowires will be present at variousdifferent heights above the BOX. According to an exemplary embodiment,the epitaxial layer 402 is grown to a uniform thickness T_(EPITAXY) offrom about 5 nanometers (nm) to about 20 nm, and ranges therebetween, onthe SOI layer 106.

Fins are then patterned in the epitaxial layer 402/SOI layer 106. SeeFIG. 5. Standard lithography and etching techniques may be used to formthe fins. By way of example only, a hardmask 502 can be formed on a sideof the epitaxial layer 402 opposite the SOI layer 106 with the footprintand location of the fins. With nanowire device configurations whereinthe nanowires are suspended it is oftentimes preferable to employ padsto anchor the ends of the nanowires. By way of example only, the padscan be attached at opposite ends of the nanowires forming a ladder-likeconfiguration wherein the nanowires are arranged like the rungs of aladder. See, for example, U.S. Pat. No. 8,927,397 issued to Chang etal., entitled “Diode Structure and Method for Gate All Around SiliconNanowire Technologies” (hereinafter “U.S. Pat. No. 8,927,397”), thecontents of which are incorporated by reference as if fully set forthherein. In that case, the hardmask 502 will have a corresponding ladderlike shape. See, for example, FIG. 2 of U.S. Pat. No. 8,927,397.

Using hardmask 502 as a mask, an etch (such as RIE) can then be used topattern the fins in the epitaxial layer 402/SOI layer 106. The BOX canserve as an etch stop for the fin RIE. Following the fin etch, thehardmask 502 can be removed. As shown in FIG. 5, each of the finscontains a portion of the epitaxial layer 402 and a portion of the SOIlayer 106. These portions of the epitaxial layer 402 will form thenanowires of the device.

Next, the SOI layer 106 is removed from the fins selective to theepitaxial layer 402 which releases the epitaxial layer 402 from thefins. See FIG. 6. The released epitaxial layer 402 forms the suspendednanowires 602 of the device. According to an exemplary embodiment, theSOI layer 106 is formed from SiGe and the epitaxial layer 402 is formedfrom Si, and the SOI layer 106 is removed from the fins using a chemicaletchant that exploits the lower oxidation potential of the SiGe SOIlayer 106 as compared to the Si epitaxial layer 402. Examples of such anetchant include, but are not limited to, a 1:2:3 mixture of HF:hydrogenperoxide (H₂O₂):acetic acid (CH₃COOH), or a mixture of sulfuric acid(H₂SO₄) and H₂O₂. Alternatively, SOI layer 106 can be selectivelyremoved using a dry etching process such as oxygen (O₂) plasma etchingor plasma chemistries typically used for etching. As will be describedbelow, when nanowire anchor pads are present, a portion of the SOI layer106 will remain beneath the pads.

As shown in FIG. 6, based on the (now-suspended) nanowires 602 havingbeen formed on a stepped surface of the SOI layer 106, the nanowires 602are suspended at varying heights above the BOX 104. For instance, inthis particular example, the nanowires are suspended over the BOX atthree different heights H1, H2, and H3 in regions I, II, and III of thewafer, respectively. It is notable that the designation of a particularregion of the wafer having nanowires at a particular suspension height(e.g., region I of the wafer having nanowires at the highest suspensionheight H1) is arbitrary and the highest, lowest, etc. suspension heightsof the nanowires may instead occur in other regions of the wafer. It isalso notable that for ease and clarity of depiction a single nanowire isbeing shown at each height H1, H2, and H3. This is merely an example,and embodiments are anticipated herein where multiple nanowires arepresent at a given height. See, for example, FIGS. 24 and 25, describedbelow.

According to an exemplary embodiment, once suspended by removal of theSOI layer 106, the nanowires 602 are then re-shaped, e.g., smoothed togive them an elliptical and in some cases a circular cross-sectionalshape. See FIG. 7. The smoothing of the nanowires may be performed, forexample, by annealing the nanowires in a hydrogen-containing atmosphere.Exemplary annealing temperatures are from about 600 degrees Celsius (°C.) to about 1,000° C., and a hydrogen pressure of from about 600 torrto about 700 torr may be employed. Exemplary techniques for re-shapingnanowires may be found, for example, in U.S. Pat. No. 7,884,004 issuedto Bangsaruntip et al., entitled “Maskless Process for Suspending andThinning Nanowires,” the contents of which are incorporated by referenceas if fully set forth herein. During this smoothing process, thenanowires are also thinned. According to one exemplary embodiment, thenanowires at this stage have a circular cross-sectional shape with across-sectional diameter of from about 7 nm to about 35 nm. If sodesired, it is also possible to further thin the nanowires using, e.g.,a high-temperature oxidation process. The process for thinning nanowiresusing high-temperature oxidation is described, for example, in U.S. Pat.No. 8,927,397.

As provided above, pads are typically employed to anchor the ends of thenanowires especially when the nanowires are to be suspended. FIG. 8provides an example of what a three-dimensional representation of thenanowires 602 and pads 802 might look like according to an exemplaryembodiment wherein both nanowires and pads are patterned in theepitaxial layer 402. By way of example only, FIG. 7 depicts across-sectional cut through line A-A′ (see FIG. 8), and the othercross-sectional views of this exemplary embodiment are similarlyoriented in the figures.

In the example shown in FIG. 8, three sets of nanowires 602 and pads 802are formed, one in each of region I, region II, and region III of thewafer. As provided above, based on the stepped surface of the SOI layer106, the nanowires 602 will be suspended at different heights above theBOX 104 in each of these regions. As also provided above, the depictionof a single nanowire in each region is merely exemplary, and embodimentsare anticipated herein where multiple suspended nanowires are present ineach of one or more of the regions.

In order to anchor the suspended nanowires 602 to the wafer, the pads802 must themselves be attached to the wafer. Thus, as shown in FIG. 8,a portion of the SOI layer 106 remains present beneath each of the pads802. It is notable that using the above-described etching processes torelease the nanowires 602 will also lead to some lateral etching of theSOI layer 106 beneath the pads. However, due to the relatively largermass of the SOI layer under the pads 802 as compared to under thenanowires 602, a portion of the SOI layer 106 will remain presentbeneath the pads 802 after the nanowire 602 have been fully released.

The present nanowire CMOS devices include one or more nanowiretransistors. Each nanowire transistor includes a source and a draininterconnected by one or more of the nanowires, and a gate stack atleast partially surrounding a portion of each of the nanowires. Theportions of the nanowires surrounded by the gate stacks serve as(nanowire) channels of the devices. The portions of the nanowiresextending laterally out from the gate stack and the pads serve as sourceand drain regions of the devices. The process for forming the gatestacks is now described. The term “gate stack” as used herein refers tothe sequence of gate materials deposited one on top of the other, i.e.,in a stack. In the present example, one or more of the gate stackmaterials will be deposited conformally around at least a portion ofeach of the nanowires. The amount by which one or more of the gate stackmaterials surrounds the nanowires is directly related to the suspensionheight of the nanowires. Namely, the greater the suspension height, themore space there is beneath the nanowires for the gate stack materials.The amount (i.e., thickness) of each of the gate stack materials alsofactors into the spacing beneath the nanowires.

Switching back now to a cross-sectional view, as shown in FIG. 9 thegate stack formation process begins with the blanket deposition of aconformal gate dielectric 902 on the wafer and surrounding the nanowires602. Specifically, using a conformal deposition process, such as atomiclayer deposition (ALD) or chemical vapor deposition (CVD), the conformalgate dielectric 902 is deposited to a uniform thickness T_(DIELECTRIC)on the exposed surface of the wafer (e.g., on a side of the BOX 104opposite the substrate 102) and around the nanowires 602. According toan exemplary embodiment, the conformal gate dielectric 902 is a high-κdielectric material, such as hafnium oxide (HfO₂) or lanthanum oxide(LaO₂). Further, according to an exemplary embodiment, the same gatedielectric material is deposited around each of the nanowires 602. Inthat case, a single deposition step may be employed to deposit theconformal gate dielectric 902 on the BOX 104 and around the nanowires602. However, if so desired, it is possible to vary the composition ofthe conformal gate dielectric 902 from one nanowire or set of nanowiresto another.

As shown in FIG. 9, based on the nanowires 602 having differentsuspension heights over the BOX 104, the space beneath the nanowires canaccommodate different amounts of gate stack material. For instance, inthe first and second regions of the wafer (i.e., region I and region IIof the wafer in FIG. 9), the nanowires 602 are suspended high enoughabove the surface of the BOX 104 such that when the layer of theconformal gate dielectric 902 is deposited onto the surface of the BOX104 and around the nanowires 602 to the thickness T_(DIELECTRIC) thereis still space available beneath the nanowires for gate stack materials.Another way to look at it is that following deposition of the conformalgate dielectric 902 to the thickness T_(DIELECTRIC), the conformal gatedielectric 902 deposited onto the surface of the BOX 104 does not makecontact (i.e., is in a non-contact position) with the conformal gatedielectric 902 deposited around the nanowires 602 in region I and regionII of the wafer. Namely, space is present between the conformal gatedielectric 902 deposited onto the surface of the BOX 104 and theconformal gate dielectric 902 deposited around the nanowires 602 inregion I and region II of the wafer.

By contrast, as shown in FIG. 9 following deposition of the conformalgate dielectric 902 to the thickness T_(DIELECTRIC), the conformal gatedielectric 902 deposited onto the surface of the BOX 104 makes contactwith the conformal gate dielectric 902 deposited around the nanowire(s)602 in region III of the wafer. According to the present techniques,oxide growth will be used to modulate the Vt of the transistors in thepresent nanowire CMOS device. Specifically, when there is direct contactbetween the (e.g., high-κ) gate dielectric 902 and the BOX 104, such asin region III of the present example, a greater amount of oxide growthwill occur at the interface between the conformal gate dielectric 902and the nanowire(s) 602. The BOX acts as an additional source of oxygenwhich diffuses through the high-κ gate dielectric. By contrast, inregions I and II of the present example, additional gate stack materialswill be deposited between the conformal gate dielectric 902 on the BOX104 and the conformal gate dielectric 902 on the nanowires 602. This isdue to the nanowires 602 being more highly suspended in region I andregion II of the wafer thus providing more space beneath the nanowires602 to be filled in with gate stack materials. These gate stackmaterials deposited under the nanowires 602 in region I and region II ofthe wafer will separate the conformal gate dielectric 902 (surroundingthe nanowires 602) from the BOX 104. In that case, the BOX 104 will notserve as an additional source of oxygen during growth, and the oxidegrown around the nanowires 602 in region I and region II of the waferwill be less than that in region III of the wafer (where the BOX acts anadditional source of oxygen). Namely, oxygen source species (O₂, O+, orH₂O) diffuse much faster and readily in dielectrics such as silicondioxide (SiO₂) and HfO₂ than in the other gate stack materials (such asthe gate metal, polysilicon, etc.). Thus, when all that separates thenanowires from the BOX is a dielectric such as HfO₂, then the BOX canserve as an oxygen source where oxygen species from the BOX can diffusethrough the dielectric to the nanowires. Conversely, when gate metal,polysilicon, etc. are present between the nanowires and the BOX throughwhich oxygen species cannot readily diffuse, then the additional oxygensource is not available.

From the above description it is apparent that the suspension height ofthe nanowires 602 is a factor in whether or not the conformal gatedielectric 902 contacts the BOX 104. Another important factor is thethickness of the conformal gate dielectric 902 T_(DIELECTRIC). Forinstance, a thicker T_(DIELECTRIC) increases the height by which thenanowires 602 have to be suspended over the BOX 104 to prevent contactbetween the conformal gate dielectric 902 and the BOX 104, and viceversa. Given the present description, it would be within thecapabilities of one skilled in the art to adjust the nanowire suspensionheight for a given desired (uniform) dielectric thickness to achieveeither contact or non-contact between the conformal gate dielectric 902and the BOX 104. According to an exemplary embodiment, the conformalgate dielectric 902 is deposited to a uniform thickness T_(DIELECTRIC)of from about 1 nm to about 5 nm, and ranges therebetween, on the BOX104 and surrounding the nanowires 602.

Next, a conformal gate metal layer 1002 is blanket deposited on theconformal gate dielectric 902 both on the wafer and around the nanowires602. See FIG. 10. Specifically, using a conformal deposition process,such as ALD or CVD, the conformal gate metal layer 1002 is deposited toa uniform thickness T_(METAL) on the wafer (i.e., on a side of theconformal gate dielectric 902 opposite the BOX 104) and around thenanowires 602 (i.e., on a side of the conformal gate dielectric 902opposite the nanowires 602). According to an exemplary embodiment, theconformal gate metal layer 1002 includes titanium and/or tantalum, e.g.,titanium nitride and/or tantalum nitride. Further, according to anexemplary embodiment, the same gate metal is deposited around each ofthe nanowires 602. In that case, a single deposition step may beemployed to deposit the conformal gate metal layer 1002 on the conformalgate dielectric 902 over the BOX and around the nanowires. However, ifso desired, it is possible to vary the composition of the conformal gatemetal layer 1002 from one nanowire or set of nanowires to another.

As shown in FIG. 10, based on the nanowires 602 having differentsuspension heights over the BOX 104, in the first and second regions ofthe wafer (i.e., region I and region II of the wafer) the conformal gatemetal layer 1002 (of thickness T_(METAL)) fully surrounds the nanowires602 and thereby physically separates the conformal gate dielectric 902surrounding the nanowires 602 from the underlying BOX 104. Thus inregion I and region II of the wafer the conformal gate dielectric 902 isnot in contact with (i.e., is in a non-contact position with) the BOX104. By contrast, in region III of the wafer the conformal gatedielectric 902 surrounding the nanowire(s) 602 is in contact with theconformal gate dielectric 902 on the BOX 104. Thus, in region III of thewafer there is direct physical contact between the conformal gatedielectric 902 and the BOX 104. The space beneath the nanowire(s) inthis third region (region III) of the wafer is closed off by theconformal gate dielectric 902 over the BOX and around the nanowires andthus does not permit the placement of any additional gate stackmaterials beneath the nanowire(s) in this region. Accordingly, theconformal gate metal layer 1002 in the region III of the wafer does notcompletely surround the nanowire(s) (i.e., the conformal gate metallayer 1002 in the region III of the wafer only partially surrounds thenanowire(s)). According to an exemplary embodiment, conformal gate metallayer 1002 is deposited to a uniform thickness T_(METAL) of from about 5nm to about 20 nm, and ranges therebetween, on the conformal gatedielectric 902 over the BOX and around the nanowires.

A conformal polysilicon layer 1102 is then blanket deposited on theconformal gate metal layer 1002 both on the wafer and around thenanowires 602. See FIG. 11. Specifically, using a conformal depositionprocess, such as ALD or CVD, the conformal polysilicon layer 1102 isdeposited to a uniform thickness T_(POLY-Si) on the wafer (i.e., on aside of the conformal gate metal layer 1002 opposite the conformal gatedielectric 902) and around the nanowires 602 (i.e., on a side of theconformal gate metal layer 1002 opposite the conformal gate dielectric902).

As shown in FIG. 11, based on the nanowires 602 having differentsuspension heights over the BOX 104, in the first region of the wafer(i.e., region I of the wafer) the conformal polysilicon layer 1102 (ofthickness T_(POLY-Si)) fully surrounds the nanowire(s) 602 and (alongwith the conformal gate metal layer 1002) thereby physically separatesthe conformal gate dielectric 902 surrounding the nanowire(s) 602 fromthe underlying BOX 104. In region II of the wafer, the conformal gatemetal layer 1002 alone physically separates the conformal gatedielectric 902 surrounding the nanowire(s) 602 from the underlying BOX104—i.e., the conformal polysilicon layer 1102 in the region II of thewafer only partially surrounds the nanowire(s). Thus in region I andregion II of the wafer the conformal gate dielectric 902 is not incontact with the BOX 104. By contrast, as provided above, in region IIIof the wafer there is direct physical contact between the conformal gatedielectric 902 and the BOX 104. According to an exemplary embodiment,conformal polysilicon layer 1102 is deposited to a uniform thicknessT_(POLY-Si) of from about 10 nm to about 30 nm, and ranges therebetween,on the conformal gate metal layer 1002 over the BOX and around thenanowires.

As shown in FIG. 12, an anneal in an oxygen ambient is then used to forma conformal oxide layer 1202 at the interface between the conformal gatedielectric 902 and the nanowires 602. Oxidation occurs at the interfacebetween the conformal gate dielectric and the nanowires due to therelative oxygen affinity of the gate dielectric (e.g., HfO₂) versus thesemiconductor material in the nanowires (Si, for example, steals oxygenfrom HfO₂ and makes it slightly sub stoichiometric) and metalcatalyzation of the oxide growth. Thus, the conformal oxide layer formedis an oxide of the semiconductor material in the nanowires, e.g., SiO₂for Si wires, germanium dioxide (GeO₂) or silicon germanium oxide forgermanium (Ge) or silicon germanium (SiGe) nanowires, respectively—seebelow. According to an exemplary embodiment, the anneal is performed ata temperature of from about 200° C. to about 500° C., and rangestherebetween, for a duration of from about 5 minutes to about 15minutes, and ranges therebetween. In the case where the conformal gatedielectric 902 is in direct physical contact with the BOX 104 (in regionIII of the wafer in this example), the BOX acts as an additional sourceof oxygen (i.e., in addition to that provided in the oxygen ambient)which diffuses through the conformal gate dielectric 902—resulting in agreater amount of oxide growth at the interface between the conformalgate dielectric 902 and the nanowires 602. By comparison, in the casewhere the conformal gate dielectric 902 is not in direct physicalcontact with the BOX 104 (in region I and region II of the wafer in thisexample), there is no source of oxygen other than what is provided inthe ambient—resulting in a lesser amount of oxide growth at theinterface between the conformal gate dielectric 902 and the nanowires602. As provided above, the amount of oxide growth at the interfacebetween the conformal gate dielectric 902 and the nanowires 602 affectsthe Vt of the respective transistor, i.e., the greater the amount ofoxide the lower the Vt.

The conformal oxide layer formed at the interface between the conformalgate dielectric 902 and the nanowires 602 in each of region I, regionII, and region III of the wafer is given the reference numeral 1202 a,1202 b, and 1202 c, respectively. The amount of oxide growth isquantified herein based on a thickness of the conformal oxide layerT_(OXIDE). The thickness of the conformal oxide layers 1202 a, 1202 b,and 1202 c, are labeled in FIG. 12 as T_(OXIDEa), T_(OXIDEb), andT_(OXIDEc), respectively. As shown in FIG. 12, the thickness of theconformal oxide layer 1202 c formed at the interface between theconformal gate dielectric 902 and the nanowires 602 in region III of thewafer (T_(OXIDEc)) is greater than the thickness of either the thicknessof the conformal oxide layer 1202 a formed at the interface between theconformal gate dielectric 902 and the nanowires 602 in region I of thewafer (T_(OXIDEa)) or the thickness of the conformal oxide layer 1202 bformed at the interface between the conformal gate dielectric 902 andthe nanowires 602 in region II of the wafer (T_(OXIDEb)). Since, thereis no direct physical contact between the conformal gate dielectric 902and the BOX 104 in region I and region II of the wafer, T_(OXIDEa) andT_(OXIDEb) are likely equivalent (i.e., T_(OXIDEa)=T_(OXIDEb)). See FIG.12. According to an exemplary embodiment, T_(OXIDEa) and T_(OXIDEb) areeach from about 0.1 nm to about 1 nm, and ranges therebetween; andT_(OXIDEc) is from about 0.5 nm to about 2 nm, and ranges therebetween.

As is apparent from the above description, an important factor herein isbeing able to control the suspension height of the nanowires, to therebycontrol whether or not the gate dielectric contacts the underlying BOX.The process detailed above for forming nanowires on a sacrificial (e.g.,SiGe) layer having a stepped surface is merely one possible wayanticipated herein for varying nanowire suspension height. Othertechniques are possible in accordance with the present teachings. Forexample, another exemplary embodiment is now described by way ofreference to FIGS. 13-23 wherein the BOX is undercut beneath thenanowires, and wherein the suspension height of the nanowires iscontrolled by regulating the depth of the BOX undercut.

As shown in FIG. 13, the starting structure is an SOI wafer having a SOIlayer 1306 separated from a substrate 1302 by a BOX 1304. In thisexample, the nanowires (and preferably also pads) are formed in the SOIlayer 1306. By comparison, in the example above, the SOI layer was usedas a sacrificial release layer on which the nanowires (and pads) wereformed. Thus, the SOI layer 1306 is formed from a semiconductor materialsuitable for use in the nanowires (and pads). By way of example only,suitable semiconductor materials for SOI layer 1306 include, but are notlimited to, Si, germanium (Ge), and SiGe. In one exemplary embodiment,the SOI layer 1306 is formed from Si.

Next, as shown in FIG. 14, standard lithography and etching techniquesare used to pattern nanowires 1404 and pads (not shown) in the SOI layer1306. By way of example only, a hardmask 1402 can be formed on a side ofthe SOI layer 1306 opposite the BOX 1304 with the footprint and locationof the nanowires 1404 (and pads). As provided above, the nanowires andpads preferably have a ladder-like configuration wherein the nanowiresare arranged like the rungs of a ladder. See, for example, U.S. Pat. No.8,927,397. In that case, the hardmask 502 will have a correspondingladder like shape. Using hardmask 1402 as a mask, an etch (such as RIE)can then be used to pattern nanowires 1404 (and pads) in the SOI layer1304. Following the nanowire etch, the hardmask 1402 can be removed.

The nanowires 1404 are then suspended over the BOX 1304 using an etch toremove (i.e., undercut) portions of the BOX 1304 beneath the nanowires1404. In the present example, the amount by which the BOX 1304 isundercut beneath the nanowires 1404 will be varied to thereby vary thesuspension height of the nanowires 1404 over the BOX 1304. Namely, inthe first of a series of etching steps, the BOX 1304 is undercut to thesame first depth D1 beneath each of the nanowires 1404. See FIG. 15.This etch releases the nanowires from the BOX 1304. According to anexemplary embodiment, this undercut etch of the BOX 1304 is carried outusing an isotropic etching process such as diluted hydrofluoric acid(DHF). At room temperature, a 100:1 DHF etches from about 2 nm to about3 nm of the BOX 1304 per minute. Thus, the timing of the etch can becontrolled to control by how much the BOX 1304 is recessed (undercut)beneath the nanowires 1404. In the same manner as described above, thesuspension height of the nanowires 1404 (based here on the depth of theundercut) will be used to control (for a given gate dielectricthickness) whether or not the gate dielectric is in direct physicalcontact with the BOX 1304.

Moving now to FIG. 16, the depth of the undercut of the BOX 1304 ismaintained (at the depth D1) in a first region(s) of the wafer (e.g., ina region I′ of the wafer), while the depth of the undercut of the BOX1304 is increased in one or more other regions of the wafer (e.g., in aregion II′ of the wafer). According to an exemplary embodiment, a mask1602 is formed on region I′ of the wafer. The mask 1602 will preventfurther undercut of the BOX 1304 in region I′ of the wafer. According toan exemplary embodiment, mask 1602 is formed from a conventionalphotoresist or a nitride masking material such as silicon nitride (SiN).A timed etch (e.g., in DHF) is then used to undercut the BOX 1304 to asecond depth D2 beneath the nanowires 1404 in a second region(s) (i.e.,in region II′) of the wafer. As provided above, a BOX etch in DHF can betimed to control the amount of BOX material that is removed.

At this point in the process, the nanowires 1404 are suspended at twodifferent heights over the BOX 1304. One could now move on to process(i.e., reshape) the nanowires and form the gate stacks. However, inorder to further illustrate the present process for undercutting the BOX1304 at various different depths, a third undercut etch is nowperformed. See FIG. 17. Specifically, as shown in FIG. 17 a mask 1702 isformed on region II′ of the wafer. The same masking material may be usedas for mask 1602. The mask 1702 will prevent further undercut of the BOX1304 in region II′ of the wafer. In this example it is assumed that themask 1602 previously formed on region I′ of the wafer (see FIG. 16)remains in place. A timed etch (e.g., in DHF) is then used to undercutthe BOX 1304 to a third depth D3 beneath the nanowires 1404 in a thirdregion(s) (i.e., in region III′) of the wafer. Following the undercutetch the masks 1602 and 1702 can be removed. The nanowires are nowsuspended over the BOX at three different heights H1′, H2′, and H3′ inregions I′, II′, and III′ of the wafer, respectively. See FIG. 18,described below.

The remainder of the process follows the same basic flow as outlined inthe example above. Namely, according to an exemplary embodiment, oncesuspended by undercutting the BOX 1304, the nanowires 1404 are thenre-shaped, e.g., smoothed to give them an elliptical and in some cases acircular cross-sectional shape. See FIG. 18. As provided above,smoothing of the nanowires may be performed, for example, by annealingthe nanowires in a hydrogen-containing atmosphere. Exemplary annealingtemperatures are from about 600° C. to about 1,000° C., and a hydrogenpressure of from about 600 torr to about 700 torr may be employed.During this smoothing process, the nanowires are also thinned. Accordingto one exemplary embodiment, the nanowires at this stage have a circularcross-sectional shape with a cross-sectional diameter of from about 7 nmto about 35 nm. If so desired, it is also possible to further thin thenanowires using, e.g., a high-temperature oxidation process.

As provided above, pads are typically employed to anchor the ends of thenanowires especially when the nanowires are to be suspended. FIG. 19provides an example of what a three-dimensional representation of thenanowires 1404 and pads 1902 might look like according to an exemplaryembodiment wherein both nanowires and pads are patterned in the SOIlayer 1306. By way of example only, FIG. 18 depicts a cross-sectionalcut through line B-B′ (see FIG. 19), and the other cross-sectional viewsof this exemplary embodiment are similarly oriented in the figures.

In the example shown in FIG. 19, three sets of nanowires 1404 and pads1902 are formed, one in each of region I′, region II′, and region III′of the wafer. As provided above, based on the undercuts of the BOX 1304at different depth, the nanowires 1404 will be suspended at differentheights above the BOX 1304 in each of these regions. It is notable thatthe designation of a particular region of the wafer having nanowires ata particular suspension height (e.g., region III′ of the wafer havingnanowires at the highest suspension height H3′) is arbitrary and thehighest, lowest, etc. suspension heights of the nanowires may insteadoccur in other regions of the wafer. As also provided above, thedepiction of a single nanowire in each region is merely exemplary, andembodiments are anticipated herein where multiple suspended nanowiresare present in each of one or more of the regions.

In order to anchor the suspended nanowires 1404 to the wafer, the pads1902 must themselves be attached to the wafer. Thus, as shown in FIG.19, the BOX 1304 is not fully undercut beneath the pads 1902. Namely,during the above-described undercut etch of the BOX 1304 some lateraletching of the BOX 1304 may occur beneath the pads. The amount oflateral etching is however minimal.

The process now continues with formation of the gate stacks surroundingthe nanowires 1404. As described above, the amount by which one or moreof the gate stack materials surrounds the nanowires is directly relatedto the suspension height of the nanowires. Namely, the greater thesuspension height, the more space there is beneath the nanowires for thegate stack materials. The amount (i.e., thickness) of each of the gatestack materials also factors into the spacing beneath the nanowires.

Switching back now to a cross-sectional view, as shown in FIG. 20 thegate stack formation process begins with the blanket deposition of aconformal gate dielectric 2002 on the wafer and surrounding thenanowires 1404. Specifically, using a conformal deposition process, suchas ALD or CVD, the conformal gate dielectric 2002 is deposited to auniform thickness T_(DIELECTRIC)′ on the exposed surface of the wafer(e.g., on a side of the BOX 1304 opposite the substrate 1302) and aroundthe nanowires 1404. According to an exemplary embodiment, the conformalgate dielectric 2002 is a high-κ dielectric material, such as HfO₂ orLaO₂. Further, according to an exemplary embodiment, the same gatedielectric material is deposited around each of the nanowires 1404. Inthat case, a single deposition step may be employed to deposit theconformal gate dielectric 2002 on the BOX 1304 and around the nanowires1404. However, as provided above, it is also possible to vary thecomposition of the conformal gate dielectric 2002 from one nanowire orset of nanowires to another.

As shown in FIG. 20, based on the variation in the depth of the undercutBOX resulting in the nanowires 1404 having different suspension heightsover the BOX 1304, the space beneath the nanowires can accommodatedifferent amounts of gate stack material. For instance, in the secondand third regions of the wafer (i.e., region II′ and region III′ of thewafer in FIG. 20), the nanowires 1404 are suspended high enough abovethe surface of the (undercut) BOX 1304 such that when the layer of theconformal gate dielectric 2002 is deposited onto the surface of the BOX1304 and around the nanowires 1404 to the thickness T_(DIELECTRIC)′there will still be space beneath the nanowires into which additionalgate stack materials can be deposited. Namely, following deposition ofthe conformal gate dielectric 2002 to the thickness T_(DIELECTRIC)′, theconformal gate dielectric 2002 deposited onto the surface of the BOX1304 does not make contact with (i.e., is in a non-contact positionwith) the conformal gate dielectric 2002 deposited around the nanowires1404 in region II′ and region III′ of the wafer. Namely, space ispresent between the conformal gate dielectric 2002 deposited onto thesurface of the BOX 1304 and the conformal gate dielectric 2002 depositedaround the nanowires 1404 in region II′ and region III′ of the wafer.

By contrast, as shown in FIG. 20 following deposition of the conformalgate dielectric 2002 to the thickness T_(DIELECTRIC)′, the conformalgate dielectric 2002 deposited onto the surface of the BOX 1304 makescontact with the conformal gate dielectric 2002 deposited around thenanowires 1404 in region I′ of the wafer. In the same manner asdescribed above, oxide growth will be used herein to modulate the Vt ofthe transistors in the present nanowire CMOS device. When there isdirect contact between the high-κ gate dielectric and the BOX, such asin region I′ of the present example, a greater amount of oxide growthwill occur at the interface between the conformal gate dielectric 2002and the nanowire(s) 1404. The BOX acts as an additional source of oxygenwhich diffuses through the high-κ gate dielectric. By contrast, inregions II′ and III′ additional gate stack materials will be depositedbetween the conformal gate dielectric 2002 on the BOX 1304 and theconformal gate dielectric 2002 on the nanowires 1404. This is due to thenanowires 1404 being more highly suspended over the BOX 1304 in regionII′ and region III′ of the wafer thus providing more space beneath thenanowires 1404 to be filled in with gate stack materials. These gatestack materials deposited under the nanowires 1404 in region II′ andregion III′ of the wafer will separate the conformal gate dielectric2002 (surrounding the nanowires 1404) from the BOX 1304. In that case,the BOX 1304 will not serve as an additional source of oxygen duringgrowth, and the oxide grown around the nanowires 1404 in region II′ andregion III′ of the wafer will be less than that in region I′ of thewafer (where the BOX acts an additional source of oxygen). Namely, asprovided above, oxygen source species (O₂, O+, or H₂O) diffuse muchfaster and readily in dielectrics such as silicon dioxide (SiO₂) andHfO₂ than in the other gate stack materials (such as the gate metal,polysilicon, etc.). Thus, when all that separates the nanowires from theBOX is a dielectric such as HfO₂, then the BOX can serve as an oxygensource where oxygen species from the BOX can diffuse through thedielectric to the nanowires. Conversely, when gate metal, polysilicon,etc. are present between the nanowires and the BOX through which oxygenspecies cannot readily diffuse, then the additional oxygen source is notavailable.

In addition to suspension height, another important factor affectingwhether or not the conformal gate dielectric 2002 contacts the BOX 1304is the thickness of the conformal gate dielectric 2002 T_(DIELECTRIC)′.Basically, a thicker T_(DIELECTRIC)′ increases the height by which thenanowires 1404 have to be suspended over the BOX 1304 to prevent contactbetween the conformal gate dielectric 2002 and the BOX 1304, and viceversa. According to an exemplary embodiment, the conformal gatedielectric 2002 is deposited to a uniform thickness T_(DIELECTRIC)′ offrom about 1 nm to about 5 nm, and ranges therebetween, on the BOX 1304and surrounding the nanowires 1404.

Next, a conformal gate metal layer 2102 is blanket deposited on theconformal gate dielectric 2002 both on the wafer and around thenanowires 1404. See FIG. 21. Specifically, using a conformal depositionprocess, such as ALD or CVD, the conformal gate metal layer 2102 isdeposited to a uniform thickness T_(METAL)′ on the wafer (i.e., on aside of the conformal gate dielectric 2002 opposite the BOX 1304) andaround the nanowires 1404 (i.e., on a side of the conformal gatedielectric 2002 opposite the nanowires 1404). According to an exemplaryembodiment, the conformal gate metal layer 2102 includes titanium and/ortantalum, e.g., titanium nitride and/or tantalum nitride. Further,according to an exemplary embodiment, the same gate metal is depositedaround each of the nanowires 1404. In that case, a single depositionstep may be employed to deposit the conformal gate metal layer 2102 onthe conformal gate dielectric 2002 over the BOX and around thenanowires. However, as provided above, it is also possible to vary thecomposition of the conformal gate metal layer 2102 from one nanowire orset of nanowires to another.

As shown in FIG. 21, based on the nanowires 1404 having differentsuspension heights over the (undercut) BOX 1304, in the second and thirdregions of the wafer (i.e., region II′ and region III′ of the wafer) theconformal gate metal layer 2102 (of thickness T_(METAL)′) fullysurrounds the nanowires 1404 and thereby physically separates theconformal gate dielectric 2002 surrounding the nanowires 1404 from theunderlying BOX 1304. Thus in region II′ and region III′ of the wafer theconformal gate dielectric 2002 is not in contact with the BOX 1304. Bycontrast, in region I′ of the wafer the conformal gate dielectric 2002surrounding the nanowire(s) 1404 is contact with the conformal gatedielectric 2002 on the BOX 1304. Thus, in region I′ of the wafer thereis direct physical contact between the conformal gate dielectric 2002and the BOX 1304. The space beneath the nanowire(s) in this first region(region I′) of the wafer is closed off by the conformal gate dielectric2002 over the BOX and around the nanowires and thus does not permit theplacement of any additional gate stack materials beneath the nanowire(s)in this region. Accordingly, the conformal gate metal layer 2102 in theregion I′ of the wafer does not completely surround the nanowire(s)(i.e., the conformal gate metal layer 2102 in the region I′ of the waferonly partially surrounds the nanowire(s)). According to an exemplaryembodiment, conformal gate metal layer 2102 is deposited to a uniformthickness T_(METAL)′ of from about 5 nm to about 20 nm, and rangestherebetween, on the conformal gate dielectric 2002 over the BOX andaround the nanowires.

A conformal polysilicon layer 2202 is then blanket deposited on theconformal gate metal layer 2102 both on the wafer and around thenanowires 1404. See FIG. 22. Specifically, using a conformal depositionprocess, such as ALD or CVD, the conformal polysilicon layer 2202 isdeposited to a uniform thickness T_(POLY-Si)′ on the wafer (i.e., on aside of the conformal gate metal layer 2102 opposite the conformal gatedielectric 2002) and around the nanowires 1404 (i.e., on a side of theconformal gate metal layer 2102 opposite the conformal gate dielectric2002).

As shown in FIG. 22, based on the nanowires 1404 having differentsuspension heights over the (undercut) BOX 1304, in the third region ofthe wafer (i.e., region III′ of the wafer) the conformal polysiliconlayer 2202 (of thickness T_(POLY-Si)′) fully surrounds the nanowire(s)1404 and (along with the conformal gate metal layer 2102) therebyphysically separates the conformal gate dielectric 2002 surrounding thenanowire(s) 1404 from the underlying BOX 1304. In region II′ of thewafer, the conformal gate metal layer 2102 alone physically separatesthe conformal gate dielectric 2002 surrounding the nanowire(s) 1404 fromthe underlying BOX 1304—i.e., the conformal polysilicon layer 2202 inthe region II′ of the wafer only partially surrounds the nanowire(s).Thus in region II′ and region III′ of the wafer the conformal gatedielectric 2002 is not in contact with the BOX 1304. By contrast, asprovided above, in region I′ of the wafer there is direct physicalcontact between the conformal gate dielectric 2002 and the BOX 1304.According to an exemplary embodiment, conformal polysilicon layer 2202is deposited to a uniform thickness T_(POLY-Si)′ of from about 10 nm toabout 30 nm, and ranges therebetween, on the conformal gate metal layer2102 over the BOX and around the nanowires.

As shown in FIG. 23, an anneal in an oxygen ambient is then used to forma conformal oxide layer 2302 at the interface between the conformal gatedielectric 2002 and the nanowires 1404. Oxidation occurs at theinterface between the conformal gate dielectric and the nanowires due tothe relative oxygen affinity of the gate dielectric (e.g., HfO₂) versusthe semiconductor material in the nanowires (Si, for example, stealsoxygen from HfO₂ and makes it slightly sub stoichiometric) and metalcatalyzation of the oxide growth. Thus, the conformal oxide layer formedis an oxide of the semiconductor material in the nanowires, e.g., SiO₂for Si wires, GeO₂ for Ge nanowires, or silicon germanium oxide for SiGenanowires. As above, the anneal may be performed at a temperature offrom about 200° C. to about 500° C., and ranges therebetween, for aduration of from about 5 minutes to about 15 minutes, and rangestherebetween. In the case where the conformal gate dielectric 2002 is indirect physical contact with the BOX 1304 (in region I′ of the wafer inthis example), the BOX acts as an additional source of oxygen (i.e., inaddition to that provided in the oxygen ambient) which diffuses throughthe conformal gate dielectric 2002—resulting in a greater amount ofoxide growth at the interface between the conformal gate dielectric 2002and the nanowires 1404. By comparison, in the case where the conformalgate dielectric 2002 is not in direct physical contact with the BOX 1304(in region II′ and region III′ of the wafer in this example), there isno other source of oxygen than what is provided in the ambient—resultingin a lesser amount of oxide growth at the interface between theconformal gate dielectric 2002 and the nanowires 1404. As providedabove, the greater the amount of oxide growth the lower the Vt.

The conformal oxide layer formed at the interface between the conformalgate dielectric 2002 and the nanowires 1404 in each of region I′, regionII′, and region III′ of the wafer is given the reference numeral 2302 a,2302 b, and 2302 c, respectively. The amount of oxide growth isquantified herein based on a thickness of the conformal oxide layerT_(OXIDE)′. The thickness of the conformal oxide layers 2302 a, 2302 b,and 2302 c, are labeled in FIG. 23 as T_(OXIDEa)′, T_(OXIDEb)′, andT_(OXIDEc)′, respectively. As shown in FIG. 23, the thickness of theconformal oxide layer 2302 a formed at the interface between theconformal gate dielectric 2002 and the nanowires 1404 in region I′ ofthe wafer (T_(OXIDEa)′) is greater than the thickness of either thethickness of the conformal oxide layer 2302 b formed at the interfacebetween the conformal gate dielectric 2002 and the nanowires 1404 inregion II′ of the wafer (T_(OXIDEb)′) or the thickness of the conformaloxide layer 2302 c formed at the interface between the conformal gatedielectric 2002 and the nanowires 1404 in region III′ of the wafer(T_(OXIDEc)′). Since, there is no direct physical contact between theconformal gate dielectric 2002 and the BOX 1304 in region II′ and regionIII′ of the wafer, T_(OXIDEb)′ and T_(OXIDEc)′ are likely the same(i.e., T_(OXIDEb)′=T_(OXIDEc)′). See FIG. 23. According to an exemplaryembodiment, T_(OXIDEb)′ and T_(OXIDEc)′ are each from about 0.1 nm toabout 1 nm, and ranges therebetween; and T_(OXIDEa)′ is from about 0.5nm to about 2 nm, and ranges therebetween.

As provided above, the depiction of a single nanowire in each region ofthe wafer in the above examples is merely exemplary. Embodiments areanticipated herein where each transistor device contains multiplenanowires at a given suspension height. By way of example only, FIG. 24illustrates one of the present transistor devices having multiplenanowires with a suspension height over the BOX so as to permit theinclusion of gate stack materials between the nanowires and the BOX,thus preventing the gate dielectric from being in contact with the BOX.Accordingly, as shown in FIG. 24, during oxide growth only a minimalamount of oxide is formed at the interface between the gate dielectricand the nanowires. The nanowires depicted in FIG. 24 are representativeof the nanowires having the highest suspension height over the BOX, suchas those in region I of the preceding stepped SOI layer example, orthose in region III′ in the preceding BOX undercut example.

By contrast, FIG. 25 illustrates one of the present transistor deviceshaving multiple nanowires with a suspension height over the BOX thatexcludes all but the gate dielectric from being present between thenanowires and the BOX. In this case, the gate dielectric is in directphysical contact with the BOX and can act as an additional oxygensource. Accordingly, as shown in FIG. 25, during oxide growth a greateramount of oxide is formed at the interface between the gate dielectricand the nanowires. Compare FIG. 24 and FIG. 25. The nanowires depictedin FIG. 25 are representative of the nanowires having the lowestsuspension height over the BOX, such as those in region III of thepreceding stepped SOI layer example or those in region I′ in thepreceding BOX undercut example.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A complementary metal oxide semiconductor (CMOS)device, comprising: nanowires suspended over a buried oxide (BOX),wherein a first one or more of the nanowires are suspended at a firstsuspension height over the BOX and a second one or more of the nanowiresare suspended at a second suspension height over the BOX, and whereinthe first suspension height is greater than the second suspensionheight; a conformal gate dielectric on the BOX and around the nanowires,wherein the conformal gate dielectric on the BOX is i) in a non-contactposition with the conformal gate dielectric around the first one or moreof the nanowires, and ii) is in direct physical contact with theconformal gate dielectric around the second one or more of thenanowires; a conformal gate metal layer on the conformal gate dielectricboth on the BOX and on the nanowires, wherein the conformal gate metallayer fully surrounds the first one or more of the nanowires but onlypartially surrounds the second one or more of the nanowires due to theconformal gate dielectric on the BOX being in direct physical contactwith the conformal gate dielectric around the second one or more of thenanowires; a conformal polysilicon layer on the conformal gate metallayer both on the BOX and on the nanowires; and a conformal oxide layerat an interface between the conformal gate dielectric and the nanowires,wherein the conformal oxide layer at the interface between the conformalgate dielectric and the first one or more of the nanowires has a firstthickness and the conformal oxide layer at the interface between theconformal gate dielectric and the second one or more of the nanowireshas a second thickness, and wherein the first thickness is less than thesecond thickness.
 2. The CMOS device of claim 1, wherein the conformalgate dielectric on the BOX and around the nanowires has a uniformthickness of from about 1 nm to about 5 nm, and ranges therebetween. 3.The CMOS device of claim 1, wherein the conformal gate metal layer onthe conformal gate dielectric has a uniform thickness of from about 5 nmto about 20 nm, and ranges therebetween.
 4. The CMOS device of claim 1,wherein the conformal polysilicon layer on the conformal gate metallayer has a uniform thickness of from about 10 nm to about 30 nm, andranges therebetween.
 5. The CMOS device of claim 1, wherein the BOX isundercut beneath the nanowires such that the BOX is undercut to a firstdepth beneath the first one or more of the nanowires and the BOX isundercut to a second depth beneath the second one or more of thenanowires, and wherein the first depth is greater than the second depth.